Conjunctiva-associated lymphoid tissue in the human eye

Mucosa-associated lymphoid tissue (MALT) represents a part of the immune system located at mucosal surfaces. Its presence in the human eye is the point in question in the current study. Its occurrence, components, topography, and probable functional significance in the human conjunctiva and lacrimal drainage system were investigated.

Fifty-three complete conjunctival sacs were obtained from cadaveric eyes, prepared as flat wholemounts, stained, optically cleared, observed in total thickness, and sectioned for light microscopic histology, immunohistochemistry, and electron microscopy. Eight lacrimal sacs and adhering canaliculi were prepared accordingly.

Lymphoid tissue was mainly observed in the palpebral conjunctiva, more pronounced in the upper than in the lower lid. It occurred in different forms: 1) In all specimens, diffuse lymphoid tissue of lymphocytes and plasma cells, most of which were IgA positive, formed a thin layer in the lamina propria. The overlying epithelium produced secretory component. 2) In approximately three fifths of the conjunctival sacs, organized follicular accumulations were embedded in this layer. They had a lenticular shape, were composed of B lymphocytes, and were apically covered by lymphoepithelium. 3) Both types could be associated with the conjunctival crypts. Lymphoid tissue with similar characteristics, including secondary follicles, was also observed inside the lacrimal drainage system. High endothelial venules were present in all types of lymphoid tissue.

Human conjunctiva and lacrimal drainage system show an associated lymphoid tissue (suggesting the term conjunctiva-associated lymphoid tissue [CALT]) that contains all components necessary for a complete immune response. Expression of immunoglobulins and secretory component indicates that the conjunctiva belongs to the secretory immune system.